Allotropy or allotropism is the property of some chemical elements to exist in two or more different forms, in the same physical state
1. Two types crystalline as well as amorphous
2. Diamond and graphite are two well-known crystalline forms of carbon.
3. Amorphous: Coke, charcoal, carbon black
4. Third form of carbon known as fullerenes
5. Graphite is thermodynamically most stable allotrope of carbon and, therefore, Δf H(-) of graphite is taken as zero.